Improvement of the corrosion resistance of steels by coating the steel surface with a metal such as Zn has been heretofore conducted. Steels coated with Zn, Zn—Al, Zn—Al—Mg, Al—Si, or the like are produced presently. With respect to coatings on steels, not only corrosion resistance but also abrasion resistance, or high adhesion after processing are demanded frequently. As methods of coating steels, hot dip metal coating methods, which are suitable for mass production, are widely used.
The corrosion resistance demanded for a coating on a steel has been yearly getting higher, and a coating with a Mg content higher than before has been proposed as by Patent Literature 1 or Patent Literature 2. However, when Mg is increased from a conventional level, metals may not melt in preparing a hot dip coating bath, and dross may be generated subject to bath composition and heating condition.
Also with respect to a hot dip metal coating film, the film adhesion after processing may be impaired by formation of an interfacial alloy layer depending on the coating composition, such that a processing method may incur restriction. Especially in the case of a non-equilibrium phase, or a film having deposited an intermetallic compound such tendency is significant, and the proposals of Patent Literature 1 and Patent Literature 2 may similarly incur a restriction in terms of a processing method.
In contrast, with respect to techniques of immersion coating (dipping), thermal spraying, or vapor deposition, since they can be applied to coating of products after processing, they are known as methods of coating hard-to-process alloys. Among them vapor deposition methods, by which steels are not dipped in molten metals, have such advantages that there is little thermal influence on steels, and the allowable range of the melting point of a metal or an alloy usable for coating is broad.
Basic methods of enhancing the corrosion resistance of steels are to add Zn to coated metal layers as in the case of formation of coated metal layers, however films to which solely Zn is added may often result in a film with corrosion resistance not adequate in many applications.
Therefore, a vapor deposition method of a Mg-containing film as described in Patent Literature 3 has been proposed. According to the same, an alloy film including one, or two or more kinds selected from 5% to 30% of Mg, from 0.5 to 5% of Al, Cr, Co, Mn, Ti, and Ni, as well as Zn as the balance is deposited, which is a coating film superior in corrosion resistance. Further, a technology in which a coating monolayer is formed by vapor deposition and thereafter a Zn—Mg coating is formed by a heat treatment has been proposed as described in Patent Literature 4.
Further, Patent Literature 5 proposes “a zinc alloy coated metal material superior in corrosion resistance and processability characterized in that a vapor deposited coated metal layer including Al at from 5 to 70% (weight-%, the same applies hereinafter), as well as one, or two or more kinds selected from Cr, Co, Ti, Ni, and Mg in total from 0.5 to 5% (provided that Ti, and Mg are less than 5%), wherein the balance is substantially Zn, is formed on a surface of metallic equipment”.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2008-255464    Patent Literature 2: JP-A No. 2011-190507    Patent Literature 3: JP-A No. H01-021066    Patent Literature 4: JP-A No. H07-268604    Patent Literature 5: JP-A No. H01-21064